As a transmission for small-scaled working vehicles, there is known from U.S. Pat. No. 4,579,183 a transmission in which power take-off shafts for taking-off auxiliary implement-driving power are journalled in an axle casing having left and right rear wheel axles and in which a hydrostatic transmission assembly operable to change the vehicle travel speed non-stepwisely is mounted on a front of the axle casing such that a space is provided before the casing at a location below the hydrostatic transmission. The hydrostatic transmission assembly includes a displacement-variable hydraulic pump of a higher level and a displacement-fixed hydraulic motor of a lower level. Motor shaft of the hydraulic motor is used for transmitting vehicle-driving power to a mechanical transmission mechanism disposed within the axle casing for driving the wheel axles, while pump shaft is used for transmitting auxiliary implement-driving power to a transmission mechanism disposed within the axle casing for driving the power take-off shafts. In this transmission, the hydrostatic transmission which may be small-sized provides the space, set forth above, before a lower portion of the axle casing, so that such space is used for projecting a front wheel-driving shaft and mid-PTO (power take-off) shaft forwardly from an inside of the axle casing.
As shown in 3P, U No. 63-179239, there is known a vehicle transmission in which a transmission assembly comprising a mechanical speed-change mechanism is mounted, in place of the hydrostatic transmission assembly set forth above, on a front of an axle casing within which both of the transmission mechanisms for driving wheel axles and for driving power take-off shafts are disposed. That is, though a mechanical speed-change mechanism is disadvantageous due to its stepwise speed-change operation as compared to a hydrostatic transmission capable of non-stepwise speed-change operation, such mechanical speed-change mechanism is superior to a hydrostatic transmission in energy efficiency and in cost so that it is required or at least desirable in some vehicle transmissions to employ such transmission assembly comprising a mechanical speed-change mechanism.
In the transmission assembly disclosed in JP, U No. 63-179239 referred to above, a drive shalt is disposed at a high level within a transmission casing, which is to be mounted on a front of an axle casing, and extends forwardly and rearwardly from the transmission casing so that the drive shaft takes a position corresponding to that of the pump shaft of a hydrostatic transmission assembly which is to be mounted also on a front of an axle casing. A speed-change shaft which corresponds to the motor shaft of the hydrostatic transmission assembly is disposed at a low level within the transmission casing and extends rearwardly from the transmission casing. For the purpose of providing a mechanical speed-change mechanism with keeping the vertical interval between the drive shaft and speed-change shaft equal to the vertical interval between the pump shaft and motor shaft of the hydrostatic transmission assembly, an intermediate shaft is disposed within the transmission casing at an intermediate level between the drive shaft and speed-change shalt such that it is located aside in a direction across the transmission casing and the mechanical speed-change mechanism is disposed between the intermediate shaft and speed-change shaft. A clutch which is required for a shifting operation of the mechanical speed-change mechanism is disposed within a clutch housing which is formed integral with a front end cover of the transmission casing such that the housing projects forwardly of the cover. It is fashioned that power is transmitted from the clutch to the intermediate shaft through a hollow shaft, which is rotatably mounted on the drive shaft and extends through the front end cover, and through a gear mechanism. Shift members of the mechanical speed-change mechanism are disposed on the speed-change shaft, and shifter forks for shifting the shift members are supported by support shafts disposed at an uppermost level within the transmission casing and extend downwardly at one side of the intermediate shaft so that they engage at their lower ends with the shift members.
Although the transmission assembly disclosed in JP, U No. 63-179239 and having the structure detailed above is well devised as the one which is to be used as a substitute for a hydrostatic transmission, some improvement is still required. That is, the clutch disposed within a clutch housing in the front end cover of the transmission casing will make the configuration of the transmission casing complicated as a whole so that there remains a problem in manufacturing and handling of the casing. Because this clutch is disposed on a front end portion of the drive shaft, load caused by clutch-disengaging operations and applied to the transmission casing will induce a large reaction force at the interlace between the transmission casing and axle casing. Consequently, a strain may be caused at such interface where power is transmitted from the inside of the transmission casing into the axle casing. Furthermore, the single mechanical speed-change mechanism disposed between the intermediate shaft and speed-change shaft provides relatively small number of change ratios so that it is considerably disadvantageous in speed-changing function as compared to a hydrostatic transmission performing non-stepwise changing operation.
In the transmission assembly set forth above, the drive shalt is used at the inside of the transmission casing only to transmit power rearward. The present invention aims at solving the problems set forth above by positively using such drive shaft.
Accordingly, a primary object of the present invention is to provide an improved transmission assembly in which the configuration of a transmission casing is simplified as a whole, the influence of load caused by clutch-disengaging operations is largely reduced and the number of speed-change ratios is greatly increased.